In the preparation of urethane-type polymers by reacting a polyisocyanate with an active hydrogen containing compound, it is known that the incorporation of trimerized isocyanate into the polymer can provide it with beneficial properties such as flame resistance, high tensile strength and flexural modulus. In flexible polymeric foams the use of a trimerized isocyanate can improve the compression set and indentation force deflection obtained.
A significant problem with the use of trimerized isocyanates is that the trimer is insoluble in the isocyanate used in preparing the polymer. Thus it is difficult to prepare and store trimerized isocyanates for use in preparing polymers. Also, it is difficult to uniformly disperse the trimer throughout the polymer due to its insolubility in the polyisocyanate.
In U.K. Pat. No. 1,337,659 it is taught to prepare solutions of trimerized isocyanates in a polyisocyanate by reacting a polyisocyanate with a short chain polyol, partially trimerizing the resulting prepolymer and diluting the product with additional polyisocyanate. Among the trimers disclosed in the patent is one prepared by cotrimerizing 4,4'-diphenylmethane diisocyanate (MDI) and toluene diisocyanate (TDI). Although improved solubility of the trimer in a polyisocyanate is achieved in this manner, it would be desirable to further improve the solubility of a trimerized isocyanate in a polyisocyanate.